The feedstock to produce calcined petroleum coke is called green coke. There are many different qualities of green coke produced throughout the world. Such green coke is a byproduct of the refining of crude oil.
Calcined petroleum coke has to meet specific quality requirements to be usable in the aluminum industry, especially for the manufacture and consumption of anodes in the reduction process. The green coke feedstock has to meet quality requirements that will result in the desired quality requirements of the final calcined coke product (anode grade coke). Therefore, not all green cokes will be suitable for the production of anode grade coke.
The most important and also widely recognized quality requirements for anode grade coke are vibrated bulk density, real density, grain stability and impurities especially metallic (listed below). Metallic impurities have a negative impact on the purity of the metal (aluminum) produced with the anodes and also on the production cost through their strong catalytic influence on the consumption of carbon needed to produce a given quality of aluminum. Calcination of green coke does not change some key quality parameters, especially the chemical elements. Some green cokes meet the quality requirements and are used to make anode grade coke. These green cokes are called "anode grade green cokes". Some green cokes will not meet the quality requirements and are used in basic applications, including but not limited to fuel in power plants. Green cokes which make up the majority of the production throughout the world are called fuel grade green cokes and are typically not calcined.
Time and technological experience have established practical limits to distinguish what is and what is not an anode grade green coke. More than 11% weight maximum of volatile matter in a green coke renders that coke unsuitable for the current state of the established calcining technology. More than 30% of the grains with a grain size greater than 4 mm makes it suitable. Less than 35% of the grains with a grain size smaller than 1 mm makes it suitable. Less than 3.5% sulfur content in the final calcined coke makes it suitable to comply with most of the current environmental regulations and less than 400 ppm vanadium in the final calcined coke makes it suitable. The above identified limits are not scientifically defined limits; they are however the accepted borders of the trade. In addition, the other quality requirements for anode grade coke (vibrated density, real density, grain stability) have an impact on which sources of green coke can be used. Commercially acceptable anode grade green cokes have the following properties:
______________________________________ Property Accepted Range ______________________________________ Sulphur (%) 0.5-3.5 Vanadium (ppm) 30-400 Nickel (ppm) 40-300 Tapped bulk 0.76-0.88 density (kg/dm.sup.3) (1 to 2 mm grain size) Real density (kg/dm.sup.3) 1.95-2.10 Grain stability (%) 60-95 (8 - 4 mm) ______________________________________
Many green cokes are not usable when they cannot provide one or several of the quality characteristics noted above required to classify the resulting calcined cokes as anode grade coke. The chemical composition and other characteristics can disqualify a green coke or a mixture of green cokes from being candidates for feedstock. The inability of some green cokes to sustain the calcining conditions can also be another major reason for disqualifying such green cokes from being candidates for feedstock. It is related to the rotary kiln calcining processes used almost exclusively throughout the world. This process is referred to as dynamic calcination.
Calcination of a green coke is the operation of applying high temperature (typically up to 1350.degree. C.) to drive out the amount of hydrocarbon volatiles remaining in the green coke and to increase the density of the carbon material. Calcination is a necessary step in the process of making anode grade coke. The dynamic calcination process relates to a situation where the green coke physically moves through a calcining device such as a rotary kiln or a rotary hearth. It enters the calciner as green coke and exits a short time later as calcined coke. In the calcining conditions that prevail in such equipment, even some green cokes that would have all the favorable characteristics to be converted into anode grade coke are not usable. The reason for this situation comes from the green coke behavior during the dynamic calcination processes that are used in the trade throughout the world. The temperature gradient in these calciners is high, up to 200.degree. C. per minute. Beyond a certain content of volatiles remaining in the green coke the coke resulting from calcination is porous, of light density. This calcined coke is referred to as "popcorn" and it is unusable for the manufacture of anodes. Consequently, otherwise very acceptable green cokes end up being used as fuel grade cokes, due to their high volatile levels, which means volatile levels in excess of 11% by weight.
Green coke is a bottom end low value by-product of the refining of crude oil. Coke chemical properties come from the original crude oil requiring the blending of various quality green cokes to obtain a green coke of average quality. Acceptable quality green cokes usable by themselves are diminishing. This is a result of the use of more sour crude oils with higher sulfur and metallic contents. The "slate of crudes" used is dictated by refinery economics rather than by quality concerns of the by-products. As stated above, this situation leads to shortages of green cokes that meet the quality requirements and environmental mandates pertaining to various coke applications.
In order to minimize this impact, it is the objective of the invention to provide a process for transforming a feedstock of green cokes, hereto generally used in the fuel grade market, into calcined coke of acceptable anode grade quality.